This invention relates generally to the control of the deflection of an electron beam in general and is particularly directed to an arrangement for providing horizontal pincushion correction for an electron beam in a raster scanned video display such as a cathode ray tube (CRT).
In deflecting an electron beam in a CRT such as a television receiver picture tube, errors arising from the geometry of the faceplate of the tube and the electron optics of the scanned beam result in pincushion distortion wherein an image appearing on the CRT's faceplate has inwardly curved top, bottom and lateral edges with the image subjected to a corresponding inwardly compressed distortion. This distortion arising from an angularly displaced beam incident upon a generally flat screen is normally corrected by appropriate adjustment of the horizontal and vertical electron beam deflection signals provided to the CRT.
Horizontal deflection control circuitry typically includes a saturable reactor for controlling or modulating the horizontal deflection current supplied to the horizontal deflection yoke of the CRT by a flyback transformer. In particular, the necessary control is effected by the horizontal deflection current in a parabolic manner so as to maximize its amplitude at the middle of each vertical scanning period and minimize its value at the top and bottom edges of the scanning period.
Pincushion distortion is known to increase with increasingly wider deflection angles of the CRT and is thus most pronounced in axially short CRT's with relatively flat screens. To compensate for the increased pincushion distortion experienced in the wide angle, shorter CRT's, the tendency is to provide increased control over the electron beam deflection current via the saturable reactor. The danger here, however, is that the core of the reactor will become saturated during the peaks of the parabolically-controlled deflection current and that control over electron beam deflection will thus be limited. The conventional approach for overcoming this problem has been to increase the size of the saturable reactor to accommodate the larger deflection currents. However, increasing the size of the saturable reactor is, of course, limited by the available space in the chassis in which the CRT is positioned as well as cost considerations. In addition, it is desirable to minimize the magnetic fields generated by the current within electromagnetic devices in electronic apparatus for reduced shielding requirements.
The prior art discloses various attempts to provide optimum electron beam control in correcting for pincushion distortion without saturating the reactor. U.S. Pat. Nos. 3,940,662 and 4,146,859 to Quirke disclose a saturable reactor device including a control coil having a current of one frequency therein, a load coil through which a current of a different frequency flows and which latter current is to be modified by the current through the control coil, and a saturable core upon which both coils are wound in an arrangement wherein the single wire windings need not be connected with one another and each winding can be wound apart from its core on bobbins that can be readily assembled axially onto the core. U.S. Pat. No. 3,444,422 to Wolber discloses a single two-window saturable reactor having a first winding connected in parallel with the horizontal deflection coil and a second winding connected in series with the vertical deflection coil and a frame deflection generator. The frame deflection current flowing in the second winding varies the inductance of the first winding so as to correct the side-to-side pincushion distortion. By phase shifting the voltage in the second winding applied to the vertical deflection coil, top-to-bottom pincushion is also corrected.
U.S. Pat. No. 3,990,030 to Chamberlin discloses a single pincushion correction transformer for both vertical and horizontal correction which includes an E core having a control winding provided around the central leg, a horizontal correction winding provided on one outer leg, and a pair of phase opposed windings on the other leg to minimize cross modulation and sensitivity to stray magnetic fields. U.S. Pat. No. 3,944,884 to Wilocki discloses a pincushion correction circuit including either a single adjustment control to adjust the correction at the beginning and end of scan or a pair of independent controls for correction at the beginning and end of scan, respectively. Each adjustment is provided by a variable inductor having a movable core, with a diode switching network switching the correction circuit to the particular variable inductor which is to control the adjustment at either the beginning or the end of scan. U.S. Pat. No. 3,854,108 to Horie et al discloses a saturable reactor having a magnetic core with right and left half portions symmetrical to one another and a center branched portion defining an air gap between one end of itself and the right and left half portions. First and second pairs of windings are coupled to the right and left half portions in series and parallel arrangements, respectively, in order to prevent saturation of the core.
The present invention represents an improvement over prior art saturable reactors of the type having a control winding responsive to a vertical rate parabolic control signal for correspondingly modulating the inductance of and the horizontal deflection current conducted by at least one secondary winding for providing side pincushion correction. In particular, a circuit is disclosed for preventing saturation of the reactor to allow for adequate control of the horizontal deflection current. This approach provides a low cost, efficient arrangement for pincushion correction which does not necessitate an increase in the size of the saturable reactor or involve an increase in the magnitude of the electromagnetic fields produced therein.